Role of environmental antigens in B cell proliferation in the bursa of Fabricius at neonatal stage.

Effect of environment antigens on S phase entry rate of bursa cells was studied by utilizing in vivo incorporation of bromodeoxyuridine (BrdUrd). S phase entry rate of bursa cells in chick embryo was determined to be 6.4%/h, this rate increased to 10.0%/h at neonatal stage and decreased to 6.4%/h 6 weeks after hatching. Evidence is presented that isolation of the bursa from the gut before hatching suppressed S phase entry rate of bursa cells after hatching, whereas S phase entry rates of thymocytes were not affected. On tissue sections, a decrease of BrdUrd+ cells was observed in medullary areas of the isolated bursa. It is proposed that in situ stimulation of bursa cells by environmental antigens to the bursa accelerates S phase entry rates, and that medullary areas are the major sites for antigen-driven B cell proliferation after hatching.